1. Field of the Invention
The present invention relates to a hydraulic device for operating a clutch in an industrial vehicle, such as a forklift truck.
2. Description of the Related Art
In an industrial vehicle having a manually operable transmission with shift changeable gears, such as a forklift truck, a forward and reverse shift lever and a speed change shift lever are provided, to change the gears, and a hydraulically operable clutch is incorporated in the transmission. The clutch is actuated by a hydraulic oil in a clutch actuating hydraulic circuit in which pressure control valves are arranged to control the operation of the clutch to engage or disengage clutch members from each other in response to the movements of the forward and reverse shift lever and the speed change shift lever.
A typical conventional hydraulic circuit for operating a clutch is shown in FIG. 14 of the attached drawings, in which a clutch actuating hydraulic circuit extends between an oil pump 204 and clutch actuating hydraulic cylinder 208 for actuating the clutch (not shown). A pressure regulating valve 205, a pressure modulating valve 209, a forward and reverse gate valve 206, a high and low speed change gate valve 207, and an accumulator 210 are arranged in the circuit, and an oil tank 215 is provided. A forward and reverse shift lever 201 and a speed change shift lever 211 are provided in the vehicle for the operation of a transmission (not shown). The forward and reverse shift lever 201 is mechanically connected to a forward and reverse shift fork 202 which engages shift changeable gears in the transmission (not shown), and is incorporated in a forward and reverse shift valve 203 which, in turn, is hydraulically connected to the forward and reverse gate valve 206, whereby the forward and reverse shift fork 202 moves to change the gears and the forward and reverse shift valve 203 changes from a neutral, port closed position to a port open position, in response to the movement of the forward and reverse shift lever 201. Thus the forward and reverse gate valve 206 opens after the forward and reverse shift valve 203 is opened, and the hydraulic oil is admitted through the clutch actuating hydraulic circuit from the oil pump 204 to the clutch actuating hydraulic cylinder 208 (if the speed change gate valve 207 is opened), to thereby engage clutch members of the clutch in accordance with a rise of the pressure of the hydraulic oil in the clutch actuating hydraulic circuit. The rise of the pressure of the hydraulic oil in the clutch actuating hydraulic circuit is regulated by the pressure modulating valve 209 and the accumulator 210.
Also, the speed change shift lever 211 is connected to a speed change valve 211 which, in turn, is hydraulically connected to a speed change shift valve 213. The speed change shift valve 213 is mechanically connected to a speed change shift fork 214, and hydraulically connected to the speed change gate valve 207, whereby the speed change shift fork 204 moves to change the gears in response to the movement of the speed change shift lever 211, and then the speed change shift valve 203 allows the hydraulic oil to flow therethrough to the speed change gate valve 207, allowing the hydraulic oil to be admitted to the clutch actuating hydraulic cylinder 208 through the clutch actuating hydraulic circuit (if the forward and reverse gate valve 206 is opened), to thereby engage clutch members of the clutch as in the above-described forward and reverse shift operation.
When one of the forward and reverse shift lever 201 and the speed change shift lever 211 is at the neutral position, and the gears are disengaged, the associated one of the gate valves 206 and 207 is returned to the initial position in which the clutch actuating hydraulic circuit is shut down and the clutch actuating hydraulic cylinder 208 is connected to the oil tank 215, causing the clutch to be disengaged.
One of the problems of the conventional hydraulic device is that, when the once-engaged gears are disengaged by moving the forward and reverse shift lever 201 or the speed change shift lever 211 to the respective neutral position, the arrangement of the hydraulic device is such that the clutch is disconnected after the gears are disengaged. This is because the forward and reverse shift lever 201 and the speed change shift lever 211 for operating the gears are mechanically connected to the movable valve elements of the shift valves 203 and 213 controlling the gate valves 206 and 207, respectively, and the ports of the respective shift valves 203 and 213 are opened by the moving valve elements. Thus the clutch is disengaged while the gears are being disengaged, and the gears may clash with each other.
Another problem of the conventional hydraulic device is that the pressure of the hydraulic oil admitted to the clutch actuating hydraulic cylinder 208 rises constantly, i.e., the rise of the pressure of the hydraulic oil is constantly regulated by the pressure modulating valve 209 and the accumulator 210 when either the vehicle is started or a shift change is carried out. Regarding the time of the rise of the pressure of the hydraulic oil, preferably the time of the rise of the pressure is prolonged, so that the pressure rises gradually to mitigate shock when the vehicle starts. This means, however, that the connection of the clutch when the gear change is carried out while the vehicle is travelling is delayed, and a relatively long time lag occurs. On the contrary, if the time of the rise of the pressure is shortened, to reduce this time lag, the shock occurring when the vehicle starts will become extensive.